When discussing topics that involve atomic and nuclear phenomena, it is useful to know the conventions for referring to features of atoms and nuclei, including the following.

  • Atomic number Z is the number of protons and number of electrons in an atom.

  • Atomic mass number A is the number of nucleons in an atom, that is, number of protons Z plus number of neutrons N in an atom; A = Z + N.

  • Atomic mass is expressed in atomic mass units u, where 1 u is equal to 1/12th of the mass of the carbon-12 atom. The atomic mass is smaller than the sum of individual masses of constituent particles because of the intrinsic energy associated with binding the particles (nucleons) within the nucleus.

In nuclear physics, a nucleus X is designated as where A is the atomic mass number and Z the atomic number. For example, the cobalt-60 nucleus is identified as ; the radium-226 nucleus as Because both Z and the chemical symbol uniquely identify the element, Z is commonly omitted leaving 60Co and 226Ra.

An element may be composed of atoms that all have the same number of protons, that is, have the same atomic number Z, but have different numbers of neutrons, that is, have different atomic mass numbers A. Such atoms of identical atomic number Z but differing atomic mass numbers A are called isotopes of a given element.

The term isotope is often misused to designate nuclear species. For example, cobalt-60, cesium-137, and radium-226 are not isotopes, since they do not belong to the same element. Rather than isotopes, they should be referred to as nuclides. On the other hand, it is correct to state that deuterium (with nucleus called deuteron) and tritium (with nucleus called triton) are heavy isotopes of hydrogen or that cobalt-59 and cobalt-60 are isotopes of cobalt. The term radionuclide should be used to designate radioactive species; however, the term radioisotope is often used for this purpose.

If a nucleus exists in an excited state for some time, it is said to be in an isomeric (metastable) state. Isomers thus are nuclear species that have common atomic number Z and atomic mass number A. For example, technetium-99m is an isomeric state of technetium-99 and cobalt-60m is an isomeric state of cobalt-60.


Henri Becquerel discovered natural radioactivity in 1896; Pierre and Marie Curie discovered radium in 1898. These basic discoveries stimulated subsequent discoveries, such as artificial radioactivity by Frédéric and Irène Joliot-Curie in 1934 and neutron-induced fission by Otto Hahn, Fritz Strassmann, Lise Meitner, and Otto Frisch in 1939. All these discoveries are of tremendous importance in science, medicine, and industry.


Radioactivity is a process by which an unstable parent nucleus transforms spontaneously into one or several daughter nuclei. These are more stable than the parent nucleus but may still be unstable and will decay further through a chain of radioactive decays until a stable nuclear configuration is reached. Radioactive decay is usually accompanied by emission of energetic particles and/or gamma rays which together form a class of radiation that is referred to as ionizing radiation.

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